The invention herein disclosed is based upon work sponsored in part by the Electric Power Research Institute, Palo Alto, Calif.
The present invention relates to transformer cores and particularly to transformer cores wound from a strip of ferromagnetic material.
A wound core is the typical configuration utilized in high volume transformers, such as distribution transformer, as it is conducive to mechanized, mass production manufacturing techniques. Although equipment has been developed to wind a ferromagnetic core strip around and through the window of a preformed, multiturn coil to produce a core and coil assembly, the most common manufacturing procedure is to wind the core independently of the preformed coil or coils with which it will ultimately be linked. This means that the core must be formed with a joint at which the core laminations can be separated to open the core and thus accommodate insertion of the core into the coil window(s). The core is then closed to remake the joint. This procedure is commonly referred to as "lacing" the core with a coil. It is of course desirable from the standpoint of operating efficiency that the magnetic reluctance of this core joint be as low as possible. Moreover, the core joint should not unduly alter the distribution of the flux flowing through the joint region.
One common type of wound core joint is the so-called step-butt joint wherein the ends of each individual lamination are butted together. Thus the plural laminations are all concentrically arranged. The positions of these individual butt joints are typically staggered throughout the core build, and thus the overall core joint has the appearance of flights of stairs, hence the term "step". While this type of core joint is convenient to produce, it results in relatively high core losses. Moreover, since the flux in each lamination, in completing its closed loop path, prefers to cross over into adjacent laminations rather than jump the high-reluctance air gap of its butt-jointed ends, the flux density in the joint region rises above the flux density existing elsewhere in the core. As a result, the core material in the joint region can become saturated since the most economical core design calls for the operating flux density to closely approach the saturation level of the core material in order to minimize the amount of core material required. In the case of amorphous metal cores, the joint configuration becomes a significant limiting factor, as the flux saturation level of amorphous metal is approximately 75% that of silicon iron.
Another joint configuration commonly utilized in wound core constructions is a step-lap joint, wherein the ends of each lamination are lapped with each other. Again, the positions of these lap joints are typically offset or staggered repeating in stairstep fashion. This joint configuration produces an extra build-up in the cross sectional area of the core in the joint region, which appears as a bump. To avoid this bump, manufacturers have added a so-called "short sheet" to the core build each time the step pattern of lap joints is repeated. This short sheet is a partial length lamination having one of its ends butted with the overlapping end of the last lamination of one step pattern of lap joints and the other of its ends butted with the underlapping end of the first lamination of the next step lap joint pattern. The presence of these short sheets builds up the cross section of the rest of the wound core to equal the cross section of the joint region. With the presence of these short sheets, the plural laminations appear as a continuous spiral from the inside to the outside of the core. It is also characterized with lap joints of a constant lap dimension throughout the core. The step-lap core joint has a similar flux saturation limitation to that of the step-butt core joint in that the flux in the short sheets must cross over into adjacent, full length laminations in order to complete their closed loop paths. This crossover flux adds to the flux already flowing in these adjacent laminations and can drive the core material in the joint region into saturation. An additional drawback to this step-lap joint construction is the additional core material represented by the short sheets. In the case of amorphous metal cores, additional material is already required to compensate for its lower saturation level as compared with silicon iron, and thus a step-lap joint with short sheets implemented in amorphous metal represents a significant cost penalty for the sake of achieving the lower core loss characteristics afforded by this material.
It is accordingly an object of the present invention to provide an improved wound transformer core.
A further object is to provide a wound transformer core having a more efficient joint configuration.
Another object is to provide a wound transformer core of the above-character having a step-lap joint wherein the extra build-up of the core cross section in the joint region is minimized.
An additional object is to provide a transformer core of the above-character whose joint is configured such that the saturation level of the joint region is substantially equal to that of the remainder of the core.
Yet another object is to provide a wound transformer core of the above-character which is constructed to make efficient use of core material.
Another object of the present invention is to provide a method for manufacturing a wound transformer core of the above-noted character.
Other objects of the invention will in part be obvious and in part appear hereinafter.